The Structure of the Ammonitella and the Direct

THE STRUCTURE OF THE AMMONITELLA AND THE

DIRECT DEVELOPMENT OF AMMONITES

V.V. Drushchits, L.A. Doguzhayeva and I.A. Mikhaylova Moscow University

ABSTRACT: The development of ammonites, like that of all recent cephalopods, proceeded without a larval stage. The embryonic shell of the ammonitella, which emerged from the egg, consisted of a protoconch and a smooth planispiral whorl with prismatic wall microstructure. The ammoni tella possessed a caecum and a prosiphon that attached its body to the wall of the protoconch. The protoconch was separated by one, less frequently two septa with prismatic microstructure. During the infant stage of the postembryonic period the ammonitella lived a planktonic mode of life, form ing a nepionic ridge with nacreous microstructure; the nepionic line was located in front of the ridge. The end of the infant stage (ammonitella) and the beginning of the adolescent (neanic) stage are recorded by the construction of a shell with a new wall microstructure, by the appearance of sculpture and a different type of coiling in the heteromorphs.

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Paleontologists have long concerned themselves with changes in the ontogeny and phylogeny of the suture line, and in the morphology and sculpture of the shell in ammonites. These changes have been used as a basis for their biological interpretation and for the periodization of ontogeny. In the last ten years the scanning electron microscope has provided new data on ontogenetic changes in the structure of the shell wall, the septa and the septal necks. These data have instigated a new discussion on the early stages of ontogeny. Let us briefly consider some of the structural features of the shell before discussing the problem as formulated.

Ammonites had an external monomorphous or heteromorphous shell. What is understood by a monomorphous shell is any type of planispiral shell consisting of closely contacting whorls, differ ing in the degree of involuteness; lying in the same plane and not altering their structural plan in the course of ontogeny. All types of shells that deviate from monomorphous are customarily clas sified as heteromorphous; such shells, which alter their structural plan in the course of ontogeny, include planispiral shells with non-contacting whorls, straight shells consisting of one shaft or two-four contacting or non-contacting shafts connected by geniculate margins, cochlespiral shells, glomerulate shells (Nippo nites) and other types of shells (criocones, ancyclocones) . The majority of ammonites had a monomorphous shell, a minority had a heteromorphous shell. The latter in cluded the first Early Devonian ammonites, which had a heteromorphous shell in the initial period of the establishment of the entire ammonite trunk, a few Late Triassic and Jurassic ammonites and the more varied and numerous Cretaceous ammonites during the period of their flourishing, their major divergence and their adaptation to various ecological niches.

Irrespective of their shape, monomorphous and heteromorphous shells consisted of two unequal parts, a living chamber housing the body of the mollusc, and the hydrostatic chambers. The first chamber, which differed from the others in that it was spherical or ridge-shaped, has become known as the protoconch, or the initial chamber. Both names are admissible and are equally em ployed, but the first is used more often, because it is shorter. All the other chambers constitute the hydrostatic apparatus, or phragmocone, The protoconch is hermetically separated from the phragmocone by the first septum, or pro septum. The siphuncle, a special process of the posterior part of the body, the function of which was to release and absorb liquid and gas in the chambers in the construction of the currently forming septum or in vertical migrations of the mollusc, extended

through all the chambers of the phragmocone, from· the protoconch to the beginning of the living chamber.

The initial part of the shell in all the Jurassic and Cretaceous ammonites, whether monomorph ous or .heteromorphous, consisted of a protoconch and a planispiral whorl surrounding it, in the

Translated from Stroeniye ammonitelly i pryamoye razvitiye ammonitov, Paleont. Zhur., no. 2, p. 57-69. 1977, Paleont. Jour. 1977, no. 2

188 ISSN 0031-0301/77/0200-0188 $ 07. 50/0 Scripta Publishing Co. © V.V. DRUSHCHITS, L.A. DOGUZHAYEVA AND I.A. MIKHAYLOVA

final quarter of which there was the nepionic ridge, accompanied by the nepionic line. The nepionic ridge was a thickening of the nacreous layer that was sometimes 1. 5-3 times the normal wall thickness. A series of appreciable transformations is to be noted in the following parts of the shell; there are changes in wall structure 3lXl in the architectonics of the whorls, and sculpture appears. Some students therefore relate the end of the larval period to the nepionic ridge, while others relate the beginning of the postembryonic period to it.

In most Cretaceous ammonites the living chamber occupied approximately 3/4 of the final whorl. In the course of growth the body of the mollusc enlarged, new portions of shell were constructed around the aperture, and at some point the body was drawn forward and cut off from the phragmo cone by the next septum to be formed. There is some uncertainty as to how this process took place and how frequently septa were constructed. Let us assume that one septum was constructed in a lunar month (Ivanov, 1971; Ivanov and Stumbur, 1975) .

Among the modern cephalopods Nautilus might serve as a model for comparison; like the am monites it has an. external shell, but it differs from ammonites in the anatomy of the soft parts, in lacking a prosiphon and a segregated caecum and in the median position of the siphuncle, Another model that might be employed is Spirula, which has an endogastrically coiled internal shell con sisting of 2.5 whorls. The shell of Spirula is similar to that of ammonites in having a spherical protoconch, a caecum and a prosiphon; as in Late Devonian Clymenia the siphuncle occupies an internal marginal position. The periodicity in the construction of new septa has not been established.

We know that when modern cephalopods (Nautilus, Spirula, Sepia) construct a septum they fill the newly created hydrostatic chamber with a fluid, the salt composition of which is similar to that of seawater, but which has a different concentration of ions and cations. When construction of the new septum is complete the fluid is partly eliminated from the chamber cavity, giving way to gas (Bidder, 1962; Denton and Gilpin-Brown, 1966; Denton, Gilpin-Brown and Howarth, 1967) . The embryogeny of the present-day Nautilus and Spirula are inadequately investigated. The Nautilus female lays large eggs up to 25 mm long and 16 mm in diameter. The nauta that emerges from the egg has an embryonic shell consisting of one whorl; the body structure of the nauta is lit tle different from that of the adult animal. A small constriction to be seen at the end of the first whorl on the surface of the embryonic shell is known as the nepionic line. Subsequent accretion of the shell of the mollusk is halted on emerging from the egg; this is indicated by the nepionic ridge, which arises at the time of adaptation of the mollusk to conditions that are new to it. The reticulate sculpture disappears completely after the nepionic ridge in Nautilus 3lXl the cross-sec tional shape alters (Shimanskiy, 1948, 1962) . These morphological changes are used to establish the limits of the embryonic shell in extinct nautiloids. Young of present-day coleoids, newly emerged from the egg capsules, differ from the adult forms in body size and in the underdevelop ment of some organs (reproductive system, fins etc. ). The smallest Spirula, taken at a depth of approximately 1000 m, had a body measuring 6 mm and an internal shell consisting of a protoconch and five hydrostatic chambers ( Chun, 1975) . According to the data of Mutvei (1964) and Barskov (1973) , the shell of Spirula consists of two semi-prismatic layers (outer and inner), between which there is apparently a thin layer of periostracwn. The septa are constructed from a dorsal conchiolin membrane and a nacreous layer, while the long septal neck extending to the preceding septum has, in addition to the two layers indicated, a further semi-prismatic inner layer; where it joins the preceding septum there is a spherulitic -prismatic layer. The same layer is discovered around the mural part of the septum. The first septum is connected to the wall of the protoconch around the apertural margin and consists of two layers- nacreous and spherulitic-prismatic. The conchiolin sheath of the caecum is attached to the septal neck of the proseptwn; an organic dorsal process extends from it to the wall of the protoconch. This process serves for attachment of the siphuncle and is homologous to the ammonite prosiphon. In Spirula (Chun, 1975) the siphuncle is surrounded by epithelium, which is underlain by connective tissue; an axial bundle of parenchyma tous tissue, in which the arterial vessel and the venous lacunae (of which there are 12 to 13) are submerged, extends along the center of the siphuncle. There is a cavity between the connective tissue and the axial bundle, which may be treated as a coelomic cavity. The siphuncular epithelium is a direct continuation of the epithelium of the shell sac, but it is distinguished by the structure and shape of the cells.

The ammonite female, like the Spirula female, probably laid rumerous small eggs. The am monitella that emerged from the egg capsule had an embryonic shell consisting of a protoconch and the first whorl. In Jurassic-Cretaceous ammonitellae the living chamber occupied 3/4 of the first whorl and was separated from the �rotoconch by a proseptwn or a proseptwn and a second septum. The proseptwn was constructed after the body of the mollusc had been drawn out of the protoconch; the second septum was possibly secreted behind it. The proseptwn was inserted 'on the apertural margin of the protoconch and it compressed the posterior part of the body, forming the caecum (fig. 1) . The prosiphon was formed from the outer layers of the mantle. The shell of the am monitella reached a diameter of 1. 5 mm, which signifies that the eggs laid by the ammonite female

189 PALEONTOLOGICAL JOURNAL

ap were probably small. No egg batches have as yet been discovered, although Wetzel ( 1959) has de scribed ammonite "larvae" from Jurassic deposits in the FGR and a chitinous membrane 0.6 mm in diameter, similar to an egg membrane. Information on the anatomy of the soft parts of the body and of the embryonic shells of extinct cephalopods may be found in works by Ruzhentsev and Shimanskiy ( 1954) , Shimanskiy and Zhuravleva ( 1961) , Ruzhentsev (1962) and Bogoslovskiy ( 1969) . Comparatively little is known concerning the struc ture of the soft parts of ammonites. X-ray studies of pyritized ammonite remains carried out in recent years have shown that the appendages of the head consisted of eight to ten short arms ( Zeiss, 1968). Remains of the preserved stomach filled with tests of foraminifers and ostracodes, remains of the FIGURE 1. Structural diagram of the protoconch upper and lower jaws,, the ink sac and the radula, and the beginning of the phragmocone as exempli and impressions of the gills have been found in fied by the genus Jauberticeras. ammonite livi� chambers (Lehmann, 1971) . The ammonite radula had seven teeth in each row, by ap - apical part, f - flange, pch - protoconch, comparisonwith the 13 in the present-day Nautilus. oms - organic membrane of siphuncle, p - pro siphon, li - lamellar layer of third septum, The ink sac, which was pyriform, and approximately s� prpch - pnsmatic layer of protoconch wall, 1/4 the length of the living chamber, had an aperture pr1wb - prismatic layer of wall of first whorl, openi� toward the anterior margin of the living prs1 and prs2- prismatic layer of first and sec chamber; we know that Nautilus lacks an ink sac. ond septa, rsn - retrochoanitic septal neck, The existence of a small number of arms and of an sip - siphuncle, c - caecum. ink sac, taken in conjunction with the structural type of the radula, indicate that ammonites were closer in the anatomy of the soft parts to coleoids than to nautiloids. Information on the biology and anatomy of modern cephalopods and on their ontogeny, and data on the morphological changes to be observed in ammonite and Nautilus shells are used for the periodization of ontogeny, the history of which has been analyzed in detail by Ivanov ( 1971). We shall confine ourselves to examination of embryogeny and of the earliest ( nepionic) stage in the postembryonic life of ammonites.an There are currentlytwo points of view on ammonite develop ment. The adherents of one view consider that ammonites, unlike the present-day cephalopods, had a larval stage (Shimanskiy, 1954; Erben, 1964) . The present authors are of the other point of view, which assumes that ammonites developed directly, without metamorphosis ( Drushchits, 1956; Drushchits and Khiami, 1969, 1970; Ivanov, 1973; Drushchits, Doguzhayeva and Mikhay lova, 1973; Mikhaylova, 1976a).

In arguing the direct development of ammonites we shall consider below the morphology and structure of the protoconch andthe first whorl, the microstructure of their wall studied in a scan ning electron microscope, the nature of changes in the suture line in early ontogeny, the structure of the primary varix and the changes to be observed in the shell after the nepionic line. Attention is concentrated on the structure of the embryonic shell and on the first (nepionic) stage of post embryonic development, during which formation of the shell of the ammonitella was completed, a process the duration of which corresponded to the formation of the primary varix. The following, neanic stage is defined by the construction of the shell after the nepionic line. Irrespective of the shape of the shell in the adult animal, the ammonitella of Jurassic and Cretaceous ammonites had a shell consisting of a protoconch and a first planispiral whorl. Its apertural margin was defined by the position of the anterior ·edge of the primary varix. The shell diameter of the ammonitella ranges between 0.7 and 1. 4 mm in Cretaceous ammonites. Three types of protoconchs have been established in Cretaceous ammonites: spherical, in which W - D .:; 10011 (where W is the width of the protoconch, Dis its diameter), ridge-like, in which W-D = 101 - 20011, andfusiform, in which W - D 201u (Drushchits and Khiami, 1969). Proto � conch diameter ranges 0. 22 and o. 80 mm, width between 0. 33 and 1.00 mm; on average a diam eter of 0, 3-0. 6 mm and a width of 0. 4-0. 7 mm predominate. The largest protoconchs have been found in members of the family Placenticeratidae ( Mikhaylova, 1974a: D = 0, 75 mm, W = 1.00 mm) . The· smallest protoconchs are found in different groups: in the Berriasian genus Ftychophyl loceras among the Phylloceratidae ( D = 0. 22-0. 28 mm) , in the Middle Jurassic genus Eurystomi ceras (D = 0. 32-0.39 mm) and the Aptian Ptychoceras (D = 0. 28-0.29 mm) among the Lytocera tidae, and in the genus Aconeceras among the Ammonitidae (D = 0. 20-0. 35 mm) . Among the genera investigated (more than 40) only Euphylloceras, Submartinoceras and some of the

190 V.V. DRUSHCHITS, L.A. DOGUZHAYEVA AND I.A. MIKHAYLOVA

Desmoceratoidea had a spherical protoconch; I in all the others the protoconch was ridge-like. The protoconch wall is formed of a single pris matic layer (pl. V, illus. 1a-e) . Slender ara gonite crystals assembled in long prisms are oriented perpendicular to the protoconch wall. This layer peters out at the beginning of the u first whorl (pl. VI, illus. 3c, 3d) . The pris matic layer that forms the wall of the first whorl appears in the last 1/4 af the protoconch and gradually replaces its prismatic layer (pl. pv VI, illus. 3a, b; figs. 1, 2) . Birkelund and Hansen ( 1968, 1974) observed two layers in the protoconch wall of two Late a Cretaceous ammonites ( Sakhalinites and Scaph c ites) ; an inner prismatic layer and an outer FIGURE 2. Shell structure in the genus Ptychoceras: subprismatic layer; the latter was distinguished a, - protoconch, planispiral whorl, beginning of first by irregularly oriented crystals submerged in shaft, surrounded by the dorsal wall of the second and (x 175), an organic matrix. The wall af the first whorl third shafts b - relationship of the planispiral before the primary varix was constructed of whor l and the three shafts (x 35), c - general appear - prismatic crystals arranged, as in the proto ance of shell (x 1). conch, perpendicularly to the surface of the pv - primary varix, s - septum, I-III- serial numbers whorl (pl. V, illus. lf, g). The protoconch 1. was jointed to the beginning of the phragmocone of shafts; otherwise as on fig. by a slitlike aperture, through which the soft body of the mollusc was drawn out. This process probably took place slowly and gradually in the course of the construction of the first whorl.

· In Jurassic and Cretaceous ammonites the primary varix is at an angle of 270-340" to the pro septum. In shape the varix varies from elongated lenticular to swollen lenticular with an asym metric steep anterior margin (pl. V, illus. 1h, i) . The thickness of the varix is 1. 5-3 times the wall thickness of the first whorl. The structural plan of the primary varix is simple; a nacreous layer, the thickness af which increases gradually or very rapidly, appears in the final quarter of the first whorl. As this layer thickens the prismatic layer tapers away; the point at which it finally peters out corresponds to the apertural margin of the embryonic shell. The primary varix is followed by the nepionic lire. The embryonic period ended before the construction of the primary varix. Having left the egg capsule, the ammonitella transferred to a planktonic mode of life. In the course of the nepionic stage of the postembryonic period, which then began, the apertural margin of the mantle formed the nacreous layers of the primary varix. The ammonitella stage concluded with completion of the construction of the primary varix, and did not cover a further two whorls, as assumed by Ivanov (1971) . The neanic stage began with the construction of a new part of the shell of the phragmocone, the wall of which was initially constructed from the outer prismatic and nacreous layers. Later, less frequently simultaneously, a thirdlayer, the inner prismatic layer, appeared. This mor phological boundary is recognized by all students although, as noted above, the biological inter pretation of it varies. Various sculptural elements and growth lines may have appeared on the shell immediately after the primary varix. In the study of heteromorphous ammonites, which has advanced appreciably in recent years, there is undoubted confirmation that the ammonitella on emergence from the egg capsule had a planispiral embryonic shell consisting of one whorl. It is now possible not merely to suggest, but to assert that' all heteromorphs had a protoconch surrounded by a planispiral whorL Any subse quent changes that led to the formation of a heteromorphous shell as such did not extend to this stage. Taken in conjunction with other characters, this specific nature of the structure of the first whorl indicates that the embryonic development of monomorphous and heteromorphous am monites was of the same type ( Bogdanova and Mikhaylova, 1975; Drushchits and Doguzhayeva, 1976; Atabekyan and Mikhaylova, 1976) . This conclusion is ba.8(!d on a study of a llUlllber of heteromorphs having the most varied shells in the adult state. In the Early Cretaceous genus Ptychoceras the first straight shaft was con structed after the planispiral embryonic shell, followed by the second and third �hafts connected by two geniculate margins (fig. 2) . The third shaft covered the embryonic shell and thus protected it against damage. In the Early Cretaceous gems Ammonitoceras the primary varix wasfollowed by a short straight shaft, and then a gentle arc with an umbilical gape and contacting, weakly in volute subsequent whorls (fig. 3) . The initial part was of the same structural type in the genus

191 PALEONTOLOGICAL JOURNAL

1(i

PLATE V V.V. DRUSHCHITS, L.A. DOGUZHAYEVA AND I.A. MIKHAYLOVA

Paraspiticeras (Wiedmann, 1969) . In the Late Cretaceous gerus Hypoturrilites, which had a turretiform adult shell, the embryonic shell also consisted of a protoconch and a first planispiral whorl approximately 1 mm in diameter (fig. 4, e-g) . The protoconch had a diameter of 0.43 mm. The cochlespiral shell began after the primary varix. The shell tube straightens after the pri mary varix in the Late Cretaceous ge!Uls Bacu lites and becomes a straight shaft (fig. 4, a-e) . � c -PR Smith ( 1901) depicted a shell belonging to the u genus Baculites found in the Upper Cretaceous b of North America; this shell consisted of a proto conch, one whorl, a proseptum and a preserved caecum, but lacked the primary varix. From FIGURE 3. Shell structure in the genus Ammonito 1975): our point of view, this shell may be treated as ceras (from Bogdanova and Mikhaylova, a - (x 22), embryonic. The planispiral whorl of the Early protoconch (x 22), b - first planispiral whorl c - general appearance of shell (x 4). Cretaceous genus Acrioceras (fig. 5, h, i) is reproduced by Branco ( 1879-1880) . Dietl ( 1975) pr - prosuture described the protoconch and the first planispiral whorl, followed by a straight shaft, in the Middle Jurassic genus Spiroceras. The proseptum, or the first septum, differs fundamentally from the primaseptum, or second septum, and also from all subsequent septa in the trend of the corrugation in the plane of symmetry. This difference emerges with particular clarity in the outlines of the prosuture ( first suture line) and the primasuture (second suture line) : in the first case a ventral and a dorsal saddle are to be seen in the plane of symmetry, while in the second the place of the saddles is occupied by corre sponding lobes present in all the following suture lines. "The fact that Jurassic and Cretaceous ammonites have a narrow ventral saddle prompted Branco ( 1879-1880) to term this prosuture angustisellate. There are two or three lobes on either side be tween the ventral and dorsal saddles. Ventral and dorsal lobes are added to these lobes in the sec ond suture, and the total number of lobes is correspondingly five (counted only on one lateral side, with the addition of the ventral and dorsal lobes) . Study of the interrelationships between the pro suture and the second suture is a fairly complicated matter that is not considered here ( Schinde wolf, 1929, 1968; Drushchits, 1956; Ruzhentsev, 1962; Mikhaylova, 1976a, b). All Early Creta ceous ammonites have six ( Schindewolf, 1968) , which indicates a shift in the late development stages of the suture lines to earlier stages of ontogeny. According to the data of all investigators who have viewed it in the electron microscope, the proseptum has prismatic microstructure (pl. VI, illus. 1) . This is apparently connected with the formation of the first septum before differentiation of the mantle and the start of the secretion of a substance with nacreous microstructure, i.e. , during embryogeny. In some genera the micro structure of the second septum is prismatic ( Erben, Flajs, and Siehl, 1969) , while in others it is nacreous (Birkelund and Hansen, 1974; Kulicki, 1974, 1975) . In the investigated gems Cadoceras (Drushchits, Doguzhayeva and Lominadze, 1976) the proseptum is af prismatic microstructure, the second septum of nacreous microstructure (pl. VI, illus. 1) . To judge by its microstructure, the second septum was apparently secreted in the egg, in the first instance, and after emergence from it, in the second. The proseptum was constructed in the egg capsule by the epithelium of the posterior part of the body, after the body had been drawn out of the protoconch. On the dorsal side it was attached to the protoconch wall at a varying distance from its margin (fig. 5) . The first septum is similarly attached to the shell in Spirula (Mutvei, 1964; Chun, 1975) . The second septum was attached to the phragmocone wall or to the proseptum on the dorsal side, which is refiected in the convergence and contacting of the second suture and the prosuture (Branco, 1879-1880, pl. a, Ulus. 3; pl. 9, illus. 1, 3) . Partial resorption of theprismatic layer and the lack of a conchiolin membrane covering theshell cavity on the inside areto be observed on the ventral side at the attachment site of the proseptum (pl. VI, illus. 2a, b) • KEY TO PLATE V Euphylloceras velledae Michelin: 189/615: (x 1200), (x 7000), 1 - specimen a) apical part of protoconch wall b) the same c) middle part of (x 2340), (x 1270), (x 3500), protoconch wall d) wall at end of protoconch e) the same f) shell wall at start of first whorl (x 2340), g) wall before primary varix (x 1400), h) primary varix (x 350), i) detail of varix (x 1000); Northwestern Caucasus, Khokodz' River; Upper Aptian. Melchiorites sp. : 2 - specimen 189/840: a) wall at end of protoconch, one prismatic layer (x 1800), b) the same (x 6000); Northwestern Caucasus, Khokodz' River; Upper Aptian. 193 PALEONTOLOGICAL JOURNAL

2() 4

PLATE VI V.V. DRUSHCHITS, L.A. DOGUZHAYEVA AND I.A. MIKHAYLOVA

membrane covering the shell cavity on the inside are to be observed on the ventral side at the attachment site of the proseptum (pl. VI, illus. 2a, b). 0 In the opinion of Kulicki ( 1975) the pro e septum was formed before the caecum and the prosiphon. The pro septum and the proto conch were formed as follows. The proto conch was constructed in the egg, after which a subprismatic layer, from which the h ventral part of the proseptum was formed, was secreted on the ventral side of the proto conch. The next stage in ontogeny was mani fested in the further secretion of the inner prismatic layer of the protoconch, in the construction of the dorsal part of the prosep tum and the flange in a deep fold and in se c cretion of the wall of the first whorl. The following data may be advanced in support: 4. FIGURE Slell structure in Baculites, Hypoturrilites a) the dorsal part af the proseptum is sepa and Acrioceras; a-d- Baculites: a-initial portion of rated from the protocon.ch wall by a distinct 20), shell (x 20), b, c - first planispiral whorl (x d - gen boundary, b) the proseptum appears to give 7) 1901); eral appearance of shell (x (from Smith, e-g - 22), way to the prismatic layer of the first whorl Hypoturrilites: e-protoconch (x f-protoconch and on the ventral side. We have considered (x 22), g - general appearance of first planispiral whorl this instance above and were of the opinion initial whorls (x 10) (from Atabekyan and Mikhaylova, 15) 1976); h, i - Acrioceras, embryonic shell (x (from that the proseptum was attached to the ven Branco, 1879-1880). Key as in figs. 1-3. tral wall in the same way as all along its periphery. Were this not to be so, we should not see the type of prosuture to be observed in all ammonites. It is difficult to accept the opinion that it was only after formation of the proseptum that the body of the mollusc was drawn out of the protoconch cavity and that the prosiphon and caecum arose. This is contradicted by data on the interrelationship between these formations in ammonites and in Spirula. The organ that regulated the regime of the hydrostatic apparatus in ammonites is the siphuncle, which extends from the posterior part of the body (figs. 5, 6) . The siphuncle originates in the protoconch as _a small swelling, that has become known as the siphonal caecum, but more often as the caecum. The caecum is situated in the protoconch cavity or partly in the first chamber. In cross-section the caecum is usually dorsoventrally compressed. After an abrupt narrowing asso ciated with the presence of a narrow opening in the proseptum, the caecum gives way to the siphuncle proper (fig. 5) . The caecum is usually closely pressed against the ventral wall of the protoconch (pl. VI, illus. 2a, 3b, 4; fig. 5) and it corresponds in its origin to the posterior part of the body of the ammonitella. A thin calcareous sheath surrounding the caecum, which was attached to the proseptum and which probably completely isolated the protoconch cavity from the other hydrostatic chambers, has been observed in one specimen of Tetragonites. The caecum was attached to the wall of the protoconch as a distinct organic band, which has been named the "prosiphon". The length of the band varies. It may be short, equal in length to the diameter of the caecum, af medium length, equal to between one arrl two diameters af the cae cum, or long; in the latter case its length is twice the diameter of the caecum. In phylloceratids and lytoceratids the prosiphon was represented by numerous short bands (Drushchits and Dog u zhayeva, 1974, pl. IV, illus. 2c).

KEY TO PLATE VI

Cadoceras sp.: 1 - specimen 189/1131: a) commencement of siphuncle (x 400), b) flange, pro septum and second septum (x 1800), c) proseptum and second septum (x 2500); Oka River; Callovian. Acanthohoplites sp. : 2 - specimen 189/1072: a) start of siphuncle (x 360), b) interrelationship between pro septum and ventral wall (x 1200); North Caucasus; Middle and Upper Aptian. Acanthohoplites sp. : 3 189/651: 1000), 400), - specimen a) shell wall approximately at the third septum (x 'b) start of siphuncle (x c) part of caecum, pro septum and shell wall (x 600), d) interrelationship between proseptum and shell wall (x 1800); Northwestern Caucasus, Khokodz' River; Upper Aptian. Phyllopachyceras sp. : 189/783, 360); 4- specimen start of siphuncle (x Northwestern Caucasus, Khokodz' River; Upper Aptian.

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pch At the start of the spiral the siphuncle f . f � occupies a central or subcentral position, but subsequently becomes ventromarginal or dorsomarginal in Late Devonian cly meniids. The authors have recently ex amined the alteration in the position of the siphuncle and in the structure of the pr formations accompanying the siphuncle . 1wh - .. · 2 p in ontogeny and phylogeny ( Drushchits, rP ch �� a b �� Bogoslovskaya, and Doguzhayeva, 1976). · pch .A s2 pch s3 · f In contrast to the point of view de fended - by the authors, Erben and his colleagues f .· · · rsn:- : ( Erben, 1964; Erben, Flajs, and Siehl, : · · �: 1969) distinguish three periods in ammon . sup .. · . . •··• . . . . ., · . · ite ontogeny; embryonic, larval and post : orn . ..' : s . : 82 larval. A conchiolin mitriform shell . . . . was formed in the embryonic period. · . sw Layers of subprismatic microstructure arose successively at its apical end; d they tapered off to either side of the apex. c Following them the shell gland secreted fully prismatic layers that constructed FIGURE 5. Structure of the initial part of the phragrnocone the narrowed apertural part of the proto in Melchiorites (a), Jauberticeras (b), Deshayesites (c) and conch. Erben relates the "first change 250). Tetragonites, (d) (x in the growth" of the shell and the end of the embryonic period with this constriction. psn-prochoanitic septal neck, s1o s2, s3 - first, second and The that emerged from the egg cap third �epta, sw - shell wall;. other symbols as on fig. 1. larva sule had a protoconch and resembled a gastropod larva of the type of a veliger. Formation of the first whorl and what is referred to as the "proseptal layer", from which the proseptum was subsequently constructed, proceeded during the larval period. At this time the body of the larva freed the protoconch and at tached itself by means of the prosiphon to the protoconch wall. The flange was constructed on the dorsal side of the protoconch. The authors cited were of the opinion that the flange was not the dorsal part of theprotocot?-ch wall; it consisted of one or two prismatic layers that were attached inside to the "proseptal" layer, i.e., the flange was formed after creation of the proseptum. The function of the flangewas unclearto the authors cited, but it was assumed that it might serve for at tachment 6f the proseptum, or could be treated, as an ontogenetically more recent formation, a secondary dorsal margin of the protoconch (the first dorsal margin of the protoconch petered:¥:� out slightly earlier) .

The larval period was concluded by the formation of 1-1/4 whorls of the shell ( Erben reckoned from the apical part of the protoconch). The primary varix arose near it from the nacreous layer, a hiatus began in the construction of new parts of the spiral tube, and the second septum formed.

In accordance with our views, Erben's postlarval period coincides with the start of the nepionic stage. The idea developed by Erben and his colleagues had·its predecessors. Smith, who studied the ontogenetic developmeit of Baculites chicoensis, long ago ( 1901) distinguished the same three stages. According to Smith, the embryonic stage was concluded by construction of the protoconch and part of the first whorl housing the body of the young mollusc. The boundary of the living cham ber of the embryo lay between the first and second septa and was expressed by a slight constriction (fig. 4, b) . The diameter of the embryonic shell was 0. 53 mm. The first larval (or ananepionic stage was recorded by the construction of the first whorl, and by formation of the proseptum and caecum. The surface of the protoconch and the first whorl was grarulated. The second larval stage ( metanepionic) began with construction of the second septum, with alteration in the sculpture at the end of the first whorl, and with replacement of the granules by fine striae and costellae. It occupied the 1/4 of the second whorl after the nepionic line and a part of the straightened shell. Smith distinguished the end of the larval stage from the appearance of the ammonite suture line as such.

• • •

It is difficult to accept the opinion of toose who propound the larval development of ammonites. The following facts are proof of the direct development of ammonites without a larval stage:

196 V.V. DRUSHCHITS, L.A. DOGUZHAYEVA AND I.A. MIKHAYLOVA

a pv c d

FIGURE 6. Embryonic and postembryonic periods in the ontogeny of ammonites: a secretion of the protoconch, b, c - formation of the first whorl, d - shell of ammonitella. Key as in figs. 1, 2 and 5.

1. All ammonites, including the heteromorph$ known to us ( Spiroceras, Ammonitoceras, Para spiticeras, Hypoturrilites, Ptychoceras, Baculites) have an embryonic shell constructed to the same plan and consisting of a protoconch and a planispiral whorl.

2. The living chamber of the ammonitella occupied 3/4 of a whorl and was separated from the protoconch by one, less frequently two septa. The shell of a dead ammonitella in which the living chamber was separated from the protoconch by a proseptum has been preserved in Upper Creta ceous deposits (Smith, 1901) .

3. The wall of the protoconch, of the first whorl to the primary varix, the proseptum and in some instances the second septum have an identical prismatic microstructure, which is indicative of a common secretary mechanism. The above arguments testify to the direct development of ammonites without a larval stage. On emerging from the egg the ammonitella lived a planktonic mode of life like the nauta and most of the present-day cephalopod "larvae". The term "larva" is incorrectly used by biologists when de scribing the ontogeny of Recent forms, since it does not reflect the true nature of the early stages of postembryonic development (Ivanov, 1971), The protoconch of the ammonitella, which was filled with gas and, possibly, partly with liquid, was a float of relatively large diameter (fig. 6). The ratio of protoconch diameter to the diameter of the embryonic shell is 1:2 or 1:2. 5. This float maintained the yo_ung animal suspended in the water and enabled it to lead a planktonic mode of life for some time. Like the young nauta, the ammonitella constructed a primary varix in the apertural margin of the embryonic shell. Further accretion of the shell was temporarily halted in connection with adaptation to new conditions,

The end of the nepionic stage and the beginning of the neanic stage are expressed morphologically by the development of the nepionic line; after this the height of the whorl of planispiral shells is usually increased and a new type of shell appears in heteromorphous ammonites. In many forms the first sculptural elements develop after the primary varix, the wall microstructure is modified and it becomes two-layered or three -layered. On the basis of all that has been said we distinguish two periods in ammonite ontogeny, embryon ic and postembryonic. During the first period, which occurred in the ammonite egg capsule, as in present-day cephalopods, the embryonic shell was formed. The changes to be seen between the protoconch and the beginning of the phragmocone are manifested only in narrowing of the aperture and cannot be regarded as the boundary of a larval stage. Consequently, ammonites developed directly, without metamorphosis, like present-day cepha lopods. The shell of the ammonitella has the following characteristic features: a) the surface of the protoconch and the first whorl is smooth as far as the nepionic line, without any traces of sculp ture; b) the boundary of the embryonic shell in ammonites is at the nepionic line, as in the present day Nautilus, after which the shell surface and the shape of the whorls change their nature; c) like all the following septa, the proseptum was secreted by the posterior part of the mantle epithelium and it was attached to the phragmocone wall, as in the present-day Nautilus and Spirula; d) the prosuture, which reflects the corrugation of the peripheral margin of the proseptum, differs from all the subsequent suture lines in having ventral and dorsal saddles, which owed their origin to the presence of a caecum; e) the primary varix was formed in the apertural part of the embryonic shell after the mollusc emerged from the egg into the first ( nepionic) stage of postembryonic de velopment; it was formed by the nacreous layer during adaptation of the ammonitella to the new habitat and the associated interruption in accretion of the shell.

197 PALEONTOLOGICAL JOUHNAL

Significant changes in shell morphology occurred in ammonites after the primary varix, the morphological boundary between the embryonic and postembryonic periods: sculpture appeared, the wall became two-layered and subsequently three-layered, and the architectonics of the shell were modified in heteromorphs. The neanic stage begins with construction of the shell after the primary varix.

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___ 1976, INTERNAL STRUCTURE OF THE SHELL IN THE GENUS Ftychoceras D'ORBIGNY: Akad. Nauk SSSR Dokl., v. 230, no. 5, p. 1210-1213. ___ and Lominadze, T. A., 1976, PROTOCONCH AND PHRAGMO CONE STRUCTURE IN THREE CALLOVIAN AMMONITE GENERA ( Pseudocadoceras, Cadoceras, Kosmoceras) : Akad. Nauk GruzSSRSoobshch._, v. 81, 2, p. 498-500. no. ___ and Mikhaylova, I.A. , 1973, THE PLANKTONIC STAGE IN THE LIFE OF AMMONITES. In: ALL-UNION PALEONTOLOGICAL SOCIETYXIX MEETING ABSTRACTS OF PROCEEDINGS, p. 16-18.

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